The 'root cause' of the loss of the spacecraft was the failed translation of English units into metric units in a segment of groundbased, navigationrelated mission software.
– Arthur Stephenson, NASA chairman of the Mars Climate Orbiter Mission Failure Investigation Board, 1999
Units are of vital importance in every civilisation as they are an essential means of communication. Extensive use of them is made in trade and science and they also play a significant role in everyday life. However, they are in no way an attainment of modern times. Whether a caveman wanted to pose with the size of the fish he had caught or a physicist wants to determine the relaxation time of an electron gas, units are needed everywhere on all levels of technological and cultural development.
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In order to have fun with them, however, a system of units must fulfil some basic requirements:
 All people have to use the same units.
 The set of units must be minimal.
 The units must be defined as exactly as possible.
 The units should be connected with each other in such a way that formulae using them become as simple as possible.
The Earth has followed a rocky road to such a system.
A Short History of Units
For the very first units, sizes of body parts were used: Ulna^{1}(cubit), foot, thumb (inch). However people don't share the same body sizes and it was unbearable to have measured data depending on the measurer.
To overcome this, cultures began to keep standards of their units (standard lengths or bars, standard weights and so on) in temples or, later, in government buildings. The oldest known unit is the Babylonian foot from 2100 BC, which is of a length of 264.5mm. Political leaders often took such definitions very personally. For example, for several years the distance between the tip of the nose of King Henry I and his thumb was the defined British yard.
Under the rule of Charlemagne in the 9th Century AD, Europe had a uniform unit scheme, introduced in 789 AD. The subsequent disunity of Europe during the Middle Ages led to a wild assortment of completely unstandardised units, differing even from town to town. It is reported that in Baden, in southwestern Germany, in 1810 there were 112 different yards, 92 units of square measure, 65 units of volume, 183 units for cereals and 80 different pounds! This may have been fun if you wanted to cheat the farmer of your neighbouring village, however it made extraterritorial trade very difficult, and was useless for science.
But a solution had already been found, in Paris.
The SI
In 1791 Delambre and Méchain completed the measurement of the distance between Dunkerque (France) and Barcelona (Spain) which was used by a panEuropean group of scientists to estimate the length of one Earth quadrant – one quarter of the circumference of the earth. Its 10^{7}th part was called one metre. Furthermore 1/1000 cubic metre was one litre and one litre of water had a mass of one kilogram. These new units were introduced in France in 1795 and in Germany in 1868. Seven years later, in 1875, 17 countries joined the metric union, called the CGPM (Conférence Général des Poids et Mesures).
In 1960 the CGPM decided to build a unit system, consisting of six (later seven) base units and their derivatives and called it Système International d'Unités: the SI. Since then, only the definitions of the base units have been improved, besides which tinkering the SI is stateoftheart. It dominates the metric countries and international science. Its home is near Paris at the Bureau International des Poids et Mesures (BIPM).
So much for history.
Base Units of the SI
The SI developed from the metric system but today both terms can be used synonymously. It is worth mentioning that all metric countries have a different set of legal units, including some nonmetric ones. The SI, in most cases, represents the core of the metric system, though.
The science that deals with measuring quantities and definition of units is metrology. Many countries run metrological institutes and partly they work as standardising authorities in their respective domains.
In regular international meetings these institutes decide about (re)definitions of SI units. Since 1983 the seven base units have been defined as follows:
SI Base Units
Quantity  Name  Definition  Dimension 

length  metre (m)  length of path travelled by light in vacuum during 1/299,792,458 of a second  L 
time  second (s)  duration of 9,192,631,770 periods of radiation of the transition between the two hyperfine levels of the ground state of ^{133}Cs  T 
mass  kilogram (kg)  equal to the mass of the international platinumiridium kilogram prototype kept under three glass bells in a safe (with three keys given to three different people) in the cellar of the Pavillion de Breteuil in Paris.  M 
electric current  ampère (A)  constant current which, if maintained in two straight parallel conductors of infinite length, of negligible circular crosssection, and placed 1 m apart in vacuum, would produce between these conductors a force of 2·10^{−7} N (newton) per metre of length  I 
thermodynamic temperature  kelvin (K)  1/273.16 of the thermodynamic temperature of the triple point of water  K 
amount of substance  mole (mol)  amount of substance which contains as many particles as there are atoms in 12 g (gram) of ^{12}C  N 
luminous intensity  candela (cd)  luminous intensity, in a given direction, of a source that emits monochromatic radiation of 540·10^{12} Hz (hertz) that has a radiant intensity in that direction of 1/683 W/sr (watt per steradian)  P 
Derived Units of the SI
Another great convenience of the SI is that all units are either base units or simple products of them according to their dimension – no coefficients, no zero point shifts. This is called a coherent system. Therefore, it's superfluous to list any conversion factors: there aren't any.
SI Derived Units
Quantity  Dimension  Name 

frequency  T^{−1}  hertz (Hz) 
force  LMT^{−2}  newton (N) 
pressure  L^{−1}MT^{2}  pascal (Pa) 
energy  L^{2}MT^{−2}  joule (J) 
power  L^{2}MT^{−3}  watt (W) 
Photometric Units 

luminous flux  P  lumen (lm) 
illuminance  L^{−2}P  lux (lx) 
Radioactivity 

activity  T^{−1}  becquerel (Bq) 
dose equivalent  L^{2}T^{−2}  sievert (Sv) 
absorbed dose  L^{2}T^{−2}  gray (Gy) 
Electrical Units 

charge  TI  coulomb (C) 
potential difference  L^{2}MT^{−3}I^{−1}  volt (V) 
resistance  L^{2}MT^{−3}I^{−2}  ohm 
conductance  L^{−2}M^{−1}T^{3}I^{2}  siemens (S) 
capacitance  L^{−2}M^{−1}T^{4}I^{2}  farad (F) 
magnetic flux  L^{2}MT^{−2}I^{−1}  weber (Wb) 
magnetic flux density  MT^{−2}I^{−1}  tesla (T) 
inductance  L^{2}MT^{−2}I^{−2}  henry (H) 
Others 

catalytic activity  T^{−1}N  katal (kat) 
refractive power  L^{−1}  diopter (dpt) 
The disadvantage of a coherent system is that some units are tiny; the pascal for example, while others are huge like the tesla. Therefore there are a few units in use that are very closely related to SI units – namely by powers of ten. These are:
SI Related Units
Name  Value  Use 

barn (b)  10^{−28} m^{2}  cross section in atomic and nuclear physics 
gauss (G)  10^{−4} T  magnetic flux density 
bar  10^{5} Pa  pressure 
litre (l)  10^{−3} m^{3}  volume (mostly of liquids) 
tonne (t)  1000 kg  weight 
ångström (Å)  10^{−10} m  dimensions in crystallography 
micron (µ)  10^{−6} m  microscopic lengths 
fermi (fm)  10^{−15} m  lengths on subatomic scale 
Metric Prefixes
A much more general solution for titchy or gigantic units are prefixes. Placing them directly before units will change their values by powers of ten. For example, 1 nF (one nano farad) is equal to 10^{−9} F = 0.000 000 001 F.
Although there are no further rules for using prefixes, there is a sort of conventional etiquette:
 Yotta, zetta, zepto and yocto are not regarded as part of the SI.
 Use deci only with metre.
 Use centi only with litre and metre.
 Use hecto only with litre and pascal.
 Don't use deka.
Metric Prefixes
Factor  Name  Symbol   Factor  Name  Symbol 

(10^{24}  yotta  Y)   10^{−1}  deci  d 
(10^{21}  zetta  Z)   10^{−2}  centi  c 
10^{18}  exa  E   10^{−3}  milli  m 
10^{15}  peta  P   10^{−6}  mikro  µ 
10^{12}  tera  T   10^{−9}  nano  n 
10^{9}  giga  G   10^{−12}  pico  p 
10^{6}  mega  M   10^{−15}  femto  f 
10^{3}  kilo  k   10^{−18}  atto  a 
10^{2}  hecto  h   (10^{−21}  zepto  z) 
10^{1}  deka  da   (10^{−24}  yocto  y) 
In information theory the same prefixes are applied to its fundamental unit, the byte (B). However, in a majority of cases of use, 1 MB actually represents 1024·1024 B = 1,048,576 bytes, rather than a strictly accurate 1,000,000 B. Therefore some metric purists called for clear distinction. The result was published in 1998. Now 1024 B are one kibibyte (1 KiB), 1024^{2} B are one mebibyte (1 MiB) and 1024^{3} B are one gibibyte (1 GiB). This wellmeant, albeit slightly academic approach, while approved by many standardisation authorities, hasn't yet made it into everyday practice.
Minutes and Hours
Minutes and hours as measures of time are not part of the core of the SI but they are very closely linked with it, particularly in everyday use of the SI system. 60 seconds are one minute (min) and 60 minutes are one hour (h).
Most people think that 24 hours are one day, which is – again, strictly speaking – only an approximation. The length of the day is adjusted to the course of the sun, which is a little bit irregular. The same is true for the week and even more for months and years. The realm of actual standard units of measurement ends at the hour.
NonSI Units in Use
Maybe life would be too boring if all people used SI units. Sometimes nonSI units are used because they make a certain specific calculations easier, however in most cases people are simply not bold enough to break with tradition, or are reluctant to 'learn' the SI unit.
For a typical example of this type of usage, a car with 150 hp has a good deal of power – but a car with 112 kW? Well, as you might guess, they are exactly the same.
Even scientists frequently prefer nonSI units. Tradition plays a role here, too, but sometimes they have a real excuse: natural units. These are useless in practice but have the convenient effect that natural constants become equal to one, which keeps formulae and calculations simple. For example, Einstein's famous equation E=mc^{2} is reduced to a very elegant E=m.
In the following tables numbers in bold face are exact per definitionem.
Scientific and International NonSI Units
Name  Value  Dimension  Use 

astronomical unit (AU, ua)  1.49597870·10^{11} m  L  astronomy, distances within the solar system 
lightyear (ly)  0.9460530·10^{16} m  L  distance in astronomy 
parsec (pc)  3.0857·10^{16} m  L  distance in astronomy 
(typographic) point (pt)  0.3514598 mm  L  dimensions in typography 
big point (bp)  0.3527778 mm  L  dimensions in typography 
international nautical mile (INM, NM)  1.852 km  L  ocean navigation 
knot (kn)  1 INM/h = 0.5144 m/s  LT^{−1}  speed of ships 
register ton (reg to, RT)  2.831685 m^{3}  L^{3}  water displacement of ships 
are (a)  100 m^{2}  L^{2}  area of farmland 
hectare (ha)  10000 m^{2}  L^{2}  area of farmland 
electronvolt (eV)  1.6021892·10^{−19} J  ML^{2}T^{−2}  energy in solid state and particle physics 
calorie (cal)  4.1868 J  ML^{2}T^{−2}  energy, calorific value of food^{2} 
tons equivalent hard coal (TET)  2.93076·10^{10} J  ML^{2}T^{−2}  7 Gcal; energy 
tons equivalent petroleum (TEP)  4.1868·10^{10} J  ML^{2}T^{−2}  10 Gcal; energy 
tons equivalent TNT  4.184·10^{9} J  ML^{2}T^{−2}  1 Mcal; energy release of bombs 
atomic mass unit (u)  1.6605655·10^{−27} kg  M  mass of atoms and elementary particles 
carat  0.2 g  M  mass of gems 
denier (den)  1.111·10^{−7} kg/m  ML^{−1}  1/9 g/km; specific weight of threads 
millimetre of mercury (mmHg)  133.322 Pa  ML^{−1}T^{−2}  eg blood pressure 
torr (Torr)  133.3224 Pa  ML^{−1}T^{−2}  gas pressure 
atmosphere (atm)  1.01325·10^{5} Pa  ML^{−1}T^{−2}  gas pressure 
pounds per square inch (psi)  6895.0 Pa  ML^{−1}T^{−2}  gas pressure 
gal (Gal)  10^{−2} m/s^{2}  LT^{−2}  fall acceleration in geophysics 
ørsted (Oe)  79.5775 A/m  IL^{−1}  10^{3}/4π A/m; magnetic field strength 
curie (Ci)  3.7·10^{10} Bq  T^{−1}  radioactive activity 
rad (rd)  0.01 Gy  L^{2}T^{−2}  (radioactive) energy dose 
röntgen equivalent man (rem)  0.01 Sv  ML^{−1}T^{−2}  (radioactive) equivalent dose 
röntgen (R)  2.58·10^{−4} C/kg  IM^{−1}T  (radioactive) ion dose 
baud  1 bit/s  T^{−1}  data transmission speed (pronounce 'bohd') 
United Kingdom
To facilitate being part of the European Union, the UK was urged to abandon all imperial units. The pint as a measure of beer or milk is still legal, as is buying pizza by the inch, but you can hire the other units for a decent fine of £5000.
Practically, however, the imperial units are still widely used. A big supermarket chain tested metric units for six months, then switched back to pounds and ounces because the customers had felt 'puzzled and bemused'.
Imperial Units
Name  Value  Remarks 

Length 

nautical mile (n mile)  1.853184 km  ocean navigation 
mile  1.609344 km  80 ch = 1760 yd 
yard (yd)  91.44 cm  36 in 
foot (ft)  30.48 cm  12 in 
inch (in, '')  2.54 cm  
milliinch (mil, ''')  25.4 µm  1/1000 in 
hand  10.16 cm  4 in; height of horses 
furlong (fur)  201.168 m  220 yd; horse racing 
chain (ch)  20.1168 m  22 yd 
fathom  1.8288 m  2 yd; depth of sea 
Area 

acre  4046.86 m^{2}  4840 yd^{2} = 10 ch^{2} 
Volume 

gallon (UKgal)  4.54609 l  
pint (pt)  0.568261 l  1/8 UKgal 
fluid ounce (fl oz)  28.41304 ml  1/160 UKgal 
barrel  163.659 l  36 UKgal; for beer 
Mass (avoirdupois system) 

pound (lb)  0.45359237 kg  
ton  1.016047 t  2240 lb 
stone  6.3502932 kg  14 lb; weight of people 
ounce (oz)  28.34952 g  1/16 lb 
dram (dr)  1.771845 g  1/16 oz 
Mass of gems and precious metals 

troy ounce (oz tr)  31.1034768 g  
pennyweight (dwt)  1.55517384 g  1/20 oz tr 
Others 

poundal (pdl)  0.138255 N  1 lb·ft/s^{2}; force 
horsepower (hp)  745.700 W  power 
British thermal unit (Btu)  1.05506 kJ  energy 
therm  105.506 MJ  10^{5} Btu; energy 
USA
The USA are bottom of the metric league. Metric units have been legal national standard in the US since 1866, though nobody cares. Curiously enough, metric units are used in most US science fiction series. Hopefully they are not complete fiction. The National Institute for Standards and Technology (NIST) tries hard to enforce the use of the SI in the USA but has hitherto had little success.
Partly the following table refers to obsolete UK units.
US Units
Name  Value  Remarks 

Length 

mile (mi), yard, foot, inch, milliinch, hand, furlong, chain, rod perch, pole and fathom (fath) as in the UK 
link (li)  20.1168 cm  0.01 ch 
line  0.635 mm  1/40 in 
gauge (gg, g)  25.4 µm  1 mil 
survey foot  30.48006 cm  1200/3937 m 
Area 

rood, acre and circular inch as in the UK 
Volume of liquids 

hogshead as in the UK 
gallon (gal)  3.7854118 l  
liquid quart (liq qt)  0.9463529 l  1/4 gal 
liquid pint (liq pt)  0.4731765 l  1/8 gal 
gill  118.2941 ml  1/32 gal 
liquid ounce (liq oz)  29.57353 ml  1/128 gal 
minim (minim)  0.0616115 ml 
fluid dram (fl dr)  3.69669 ml  60 minim 
tablespoon  14.78676 ml  1/2 liq oz 
teaspoon  9.8578432 ml  1/3 liq oz 
cup  236.58824 ml  16 tablespoon 
barrel, barrel petroleum  158.9873 l  42 gal, only petroleum 
Volume of dry goods 

dry barrel (bbl)  115.6271 l  7056 in^{3} 
bushel (bu)  35.2391 l 
peck (peck)  8.80977 l  1/4 bu 
dry quart (dry qt)  1.101221 l  1/8 peck 
dry pint (dry pt)  0.550610 l  1/2 quart 
Mass 

pound, short ton, ounce, dram, grain (grain) and slug as in the UK 
long ton  1.016047 t  2240 lb 
long hundredweight (cwt)  50.8024 kg  112 lb 
short hundredweight (sh cwt)  45.3592 kg  100 lb 
Mass of gems and precious metals 

troy ounce and pennyweight as in the UK 
troy pound (lb tr)  0.37324172 kg  12 oz tr 
Mass of drugs 

apothecaries' pound (lb ap)  0.37324172 kg  1 lb tr 
apothecaries' dram (dr ap)  3.8879346 g  1 drachm 
apothecaries' scruple (s ap)  1.2959782 g  1 scruple (UK) 
Others 

poundal and horsepower as in the UK 
Germany
Since the old days in Baden the measurement situation has been greatly improved. As in most continental countries, the use of nonSI units has become very rare here. Responsible is the PTB in Braunschweig, which calls itself the 'Guardian of Units'.
German Units
Name  Value  Use  Imperial Counterpart^{3} 

Pfund  0.5 kg  esp. eatables  pound 
Zentner  50 kg  weight of goods and fat people  hundredweight 
Pferdestärke (PS)  735.49875 W  power of engines  horse power 
Peculiar Units in Use
Temperature
Temperature is basically a rather simple concept. There is a point of absolute zero temperature (0 K – ie, no heat at all) and from there you can measure temperature just like length or mass. Unfortunately this leads to very odd values for everyday temperatures – around 300 K. Therefore people use other scales with other zero points eg, °C, °F. This makes it necessary to give formulae rather than mere factors to convert between temperature units.
Let's assume you wanted to know how much Fahrenheit 20 °C is. The table says (5th column, 4th row) °F = 1.8 · C + 32'. This yields 68 °F. Voilà.
Rankine and reaumur are obsolete temperature units. The Rankine was intended to be for Fahrenheit what kelvin is for Celsius – a thermodynamic temperature scale. Celsius made the race all over the world, however, which meant the end of the rankine.
Temperature Conversion Formulae
 K =  °R =  °C =  °F =  °Re = 

kelvin (K)  —  1.8 · K  K − 273.15  1.8 · K − 459.67  0.8 · K − 218.52 
rankine (°R)  5/9 · R  —  5/9 · R − 273.15  R − 459.67  4/9 · R − 218.52 
Celsius (°C)  C + 273.15  1.8 · C + 491.67  —  1.8 · C + 32  0.8 · C 
Fahrenheit (°F)  5/9 · (F − 32) + 273.15  F + 459.67  5/9 · (F − 32)  —  4/9 · (F − 32) 
reaumur (°Re)  1.25 · Re + 273.15  2.25 · Re + 491.67  1.25 · Re  2.25 · Re + 32  — 
Historical remark: Probably you have noticed all the degree signs in the temperature units. These have nothing to do with angles of course, but with a problem of the early days of temperature measurement: What does zero temperature mean, really?
 Fahrenheit (1714) said 'temperature of a mixture of water, ice and ammoniac'.
 Reaumur (1730) said 'temperature of a mixture of water and ice'.
 Celsius (1742) said 'temperature of boiling water'. Later people found this potty and changed it to Reaumur's definition.
Because all of this was arbitrary, these units were disfigured with degree signs. The real zero point remained a mystery until W Thompson, or Lord Kelvin, determined it in 1848. Hence the kelvin has no '°' and is allowed to wear metric prefixes like mK or µK.
Pureness of Precious Metals
It is not only that outside the SI there are many units for the same quantity, sometimes even within the SI one unit can indicate multiple quantities. The carat, for example, is a unit of mass, but it can also be used to indicate how much gold, silver etc is contained in a given alloy. 25/6 · carat gives the percentage of pureness. Thus a say 22carat gold chain contains 92% gold. Pure gold has 24 carats, so higher values would be a metallurgical miracle (or simply a lie).
Logarithmic Units
A simplified conception of the WeberFechner law says that our perception of external stimulation is proportional to the logarithm of that stimulation. This is only a very rough approximation, but it explains the need for logarithmic units, especially if our senses are involved.
Mathematical note: Unfortunately the logarithm can't stand units. In order to get rid of them one has to divide the quantity by another quantity of the same unit. This second quantity is part of the definition and is usually very small to avoid negative values.
decibel (dB): Actually the decibel is a far more general measurement, but mostly it is used to indicate the loudness of sound. Let p be the sound pressure, then 10·log(p/20µPa) is the loudness in decibels. 40 dB is normal small talk, 140 dB is a jet engine and 180 dB is lethal. However our ears are not equally sensitive for all frequencies; the maximum is at approx. 1000 Hz. The phon scale takes this into account (at 1000 Hz, phon and dB are identical).
magnitudines (^{m}): Astronomers specify the brightness of stars in magnitudines (or magnitudes) which is based on an ancient Greek system. In this the brightest stars had 1^{m} and the faintest 6^{m}. Today's exact definition is m=−2.5·log(s/s_{0}). Here s indicates the illuminance of the star seen from earth and s_{0} is adjusted in such a way that the Pole Star has exactly 2.12^{m}. Later it was found that the brightness of the Pole Star is in fact slightly variable, but astronomers are very patient people.
Richter scale: It's a measure for the intensity of earthquakes. Let A be the maximal amplitude of the ground oscillation 100 km away from the epicentre of an earthquake, then log(A/1µm) is its value on the Richter scale. Earthquakes beyond 6 are regarded as really big ones, for them A can be several metres.
pH scale: A measure of acidity. Let n be the molar concentration of H^{+}ions in a solution, then −log(n/(1mol/l)) is its pH value. In water, values from 0 (strong acid) up to 14 (strong alkali) are possible. Pure water has a pH of 7 (neutral), yet drinking water is a little bit lower.
bit: The bit can be seen as a logarithmic measure of information. If a certain data container can represent N different possible contents (eg numbers) it has (3.322·log N) bit. For example, a variable that can have values from 0 up to 255 is a 3.322·log 256 = 8 bit variable. 8 bit form one byte. There are even larger units like word, double word and paragraph, however their sizes are not clearly standardised. See also the discussion of binary metric prefixes above^{4}.
Angular Units
The only mathematically legitimate unit of plane angle is the radian (rad). It's the length of the corresponding arc in the unit circle. For all nonmathematicians: 2π rad is the full angle, π/2 rad^{5} is a right angle.
For solid angle the steradian (sr) is used. It's the area of the corresponding sphere segment of the unit sphere. The full sphere has 4π sr.
There aren't actually any other sensible – or equally intuitive – possibilities to measure angles, but mankind is not very sensible and highly imaginative. One degree (°) is (π/180) rad (so 360° are the full angle). Every degree is divided into 60 arc minutes (') end every arc minute into 60 arc seconds ('').
One gon is (π/200) rad. Gon is also called grade or new degree (^{g}). Every new degree is divided into 100 new minutes (^{c})^{6} and every new minute into 100 new seconds (^{cc}). Remarkably enough the gon is a legal unit, however almost nobody uses it.
There are other angular units which are even more bizarre. All the units in the following table, for example – legal or not – must also be regarded as obsolete units, excluding degree and its derivatives.
Angular Units
Name  Value  Full Angle  Remarks 

Plane angle 

degree (°)  1.745329·10^{−2} rad  360°  π/180 rad 
(arc) minute (')  2.908882·10^{−4} rad  21600'  (1/60)° 
(arc) second ('')  4.848137·10^{−6} rad  1,296,000''  (1/3600)° 
(arc) gon (gon), grade (grade), new degree (^{g})  1.570796·10^{−2} rad  400 gon  (1/200) rad 
new minute (^{c}), cgon  1.570796·10^{−4} rad  40000^{c}  (1/100) gon 
new second (^{cc})  1.570796·10^{−6} rad  4,000,000^{cc}  (1/10000) gon 
revolution (r)  6.283185 rad  1 r  2π rad 
mil (mil), (artilleristic) point (¯)  9.817477·10^{−4} rad  6400 mil  π/3200 rad 
(nautical) point ('')  0.1963495 rad  32 point  π/16 rad; ocean navigation 
Solid angle 

square degree ((°)^{2})  3.046174·10^{−4} sr  41252.97(°)^{2}  (π/180)^{2} sr 
Obsolete Units
The following passages contain units not used anymore. However you may still come across them sometimes.
It is virtually impossible to list all units, there being almost as many potential units as there are human beings. If you encounter an unknown unit and want to look for its value, a rather good starting point is one of the national genealogy institutes, especially if it is a historical unit. Otherwise the metrological authorities are worth a try.
Obsolete Scientific and International Units
Name  Value  Dimension  Use 

xunit (KX)  1.00202·10^{−10} m  L  spectroscopy 
didot point (dd)  0.376 mm  L  continental typography 
cicero (cc)  4.531 mm  L  continental typography 
pica  4.218 mm  L  typography 
geographical mile  7.421591 km  L  
eötvös (E)  10^{−9} s^{−2}  T^{−2}  acceleration in geophysics 
franklin (Fr)  3.33564·10^{−10} C  TI  electric charge 
debye (D)  3.33564·10^{−30} Cm  LTI  electric dipole momentum 
clausius (Cl)  4.1868 J/K  ML^{2}T^{−2}K^{−1}  1 cal/K; entropy 
rutherford (Rd)  10^{6} Bq  T^{−1}  radioactive activity 
jansky (Jy)  10^{−26} J/m^{2}  MT^{−2}  irradiation 
biot (Bi)  10 A  I  electric current 
maxwell (M, Mx)  10^{−8} Wb  L^{2}MT^{−2}I^{−1}  magnetic flux 
gilbert (Gb)  0.795775 A  I  10/4π A; magnetic tension 
new candle (NK)  1 cd  P  luminous intensity 
international candle (IK)  1.019 cd  P  luminous intensity 
nit (nt)  1 cd/m^{2}  PL^{−2}  luminous density 
stilb (sb)  10^{4} cd/m^{2}  PL^{−2}  luminous density 
apostilb (asb)  0.318310 cd/m^{2}  PL^{−2}  (1/π) cd/m^{2}; luminous density 
lambert (La)  3183.10 cd/m^{2}  PL^{−2}  (1/π) cd/cm^{2}; luminous density 
phot (ph)  10^{4} lx  PL^{−2}  illuminance 
litre atmosphere (l atm)  101.325 J  ML^{2}T^{−2}  energy 
gamma (γ)  10^{−9} kg  M  mass 
pond (p)  9.80665·10^{−2} N  MLT^{−2}  force 
dyne (dyn)  10^{−5} N  MLT^{−2}  force 
poise (P)  0.1 Pa s  ML^{−1}T^{−1}  dynamic viscosity 
stokes (St)  10^{−4} m^{2}/s  L^{2}T^{−1}  kinematic viscosity 
erg  10^{−7} J  ML^{2}T^{−2}  energy 
enzyme unit (U)  1.6667·10^{−8} kat  NT^{−1}  1 µmol/minute 
United Kingdom
Obsolete Imperial Units
Name  Value  Remarks 

Length 

rod, perch, pole (rod)  5.0292 m  5.5 yd 
Area 

rood  1011.71 m^{2}  40 rod^{2} = 1/4 acre 
circular inch  5.06707 cm^{2}  (π/4) in^{2} 
Volume 

peck  9.09218 l  2 UKgal 
bushel  36.3687 l  8 UKgal 
quarter  290.9498 l  64 UKgal 
chaldron  1.309273 m^{3}  4 quarter 
quart (qt)  1.136523 l  1/4 UKgal 
gill  142.0652 ml  1/32 UKgal 
minim (min)  0.0591939 ml  1/180 fl oz 
fluid drachm (fl dr)  3.55163 ml  60 min 
fluid scruple  1.183877 ml  20 min 
hogshead (hhg)  238.4809 l  only for liquids 
Mass 

short ton  0.907185 t  2000 lb 
hundredweight (cwt)  50.8024 kg  8 stone = 112 lb 
cenral (sh cwt)  45.3592 kg  100 lb 
quarter  12.7005864 kg  1/4 cwt 
grain (gr)  64.7989 mg  1/7000 lb 
slug  14.59390 kg 
Mass of drugs 

apothecaries' ounce (oz ap)  31.1034768 g  480 gr 
drachm  3.8879346 g  60 gr 
scruple  1.2959782 g  20 gr 
Germany
The following units are all Prussian apart from one. Prussia was the northeastern part of Germany, and the industrial and scientific heartland of the country. The exception is the Deutsche Meile, a general, historical German unit.
Obsolete German Units
Name  Quantity  Value 

Deutsche Meile (mile)  length  7.5 km 

Linie (line)  length  0.218 cm 
Zoll (inch)  length  12 Linien = 2.615 cm 
Fuß (foot)  length  12 Zoll = 31.39 cm 
Elle (cubit)  length  25.5 Zoll = 66.69 cm 
Klafter  length  6 Fu = 1.883 m 
Rute (rod)  length  2 Klafter = 3.766 m 
Meile (mile)  length  2000 Ruten = 7.532 km 
Schritt (yard)  length  1/10000 Meile = 75.32 cm 
Russia
Today Russia uses the metric system in almost all measurements – even in their airplanes, which is potentially dangerous since western control towers normally use imperial units. Note that the Russian dujm and fut are exactly equal to the inch and foot. The names are mere transcriptions from Cyrillic.
Obsolete Russian Units
Name^{7}  Quantity  Value 

dujm (inch)  length  2.540 cm 
fut (foot)  length  12 dujm = 0.3048 m 
arschin (cubit)  length  7/3 fut = 71.12 cm 
milja (mile)  length  10500 arschin = 7.468 km 
funt (pound)  mass  409.512 g 
tonna (ton)  mass  4800 funt = 1965.658 kg 
Status and Future of the SI
Today, 94.5% of the world's population uses the metric system. On the whole only three countries haven't included it into their industrial standards: The USA, Myanmar and Liberia.
Most other systems have been connected to the SI. Since 1959, the inch is defined using the metre, and the avoirdupois, troy and apothecaries' systems via the kilogram.
The Convention of the Metre of 1875 is still the basis of all international agreement on units of measurement. There are now 48 member countries in the BIPM, including all major industrialised nations. However, practical enforcement of usage varies heavily from country to country.
The US National Institute of Standards and Technology (NIST) has launched a 'Metric Program' under the banner 'Toward a Metric America'. Hopefully they will succeed in converting the US to usage of metric units. However, doing this comprehensively and quickly would probably cost hundreds of billions of dollars.
Fortunately the German PhysikalischTechnische Bundesanstalt (PTB) needn't do this. They focus on more accurate definitions of units. For example the kilogram is defined in a rather anachronistic way by a prototype that is likely to change over the centuries. Their project 'Avogadro' ia an attempt at an overhaul.